Twin primes less than 87 in R - r

Question

I am trying to list the first 87 twin primes. I'm using the Eratosthenes approach. Here is what I've worked on so far
Eratosthenes <- function(n) {
# Return all prime numbers up to n (based on the sieve of Eratosthenes)
if (n >= 2) {
sieve <- seq(2, n) # initialize sieve
primes <- c() # initialize primes vector
for (i in seq(2, n)) {
if (any(sieve == i)) { # check if i is in the sieve
primes <- c(primes, i) # if so, add i to primes
sieve <- sieve[(sieve %% i) != 0] # remove multiples of i from sieve
}
}
return(primes)
} else {
stop("Input value of n should be at least 2.")
}
}
Era <- c(Eratosthenes(87))
i <- 2:86
for (i in Era){
if (Era[i]+2 == Era[i+1]){
print(c(Era[i], Era[i+1]))
}
}
First thing I dont understand is this error:
Error in if (Era[i] + 2 == Era[i + 1]) { :
missing value where TRUE/FALSE needed
Second thing is in the list there are missing twin primes so for example (29,31)

Answer 1

Within your for loop, i is not index any more but the element in Era. In this case, you can try using (i+2) %in% Era to judge if i+2 is the twin
for (i in Era){
if ((i+2) %in% Era){
print(c(i,i+2))
}
}
which gives
[1] 3 5
[1] 5 7
[1] 11 13
[1] 17 19
[1] 29 31
[1] 41 43
[1] 59 61
[1] 71 73
A simpler way might be using diff, e.g.,
i <- Era[c(diff(Era)==2,FALSE)]
print(cbind(i,j = i+2))
which gives
> print(cbind(i,j = i+2))
i j
[1,] 3 5
[2,] 5 7
[3,] 11 13
[4,] 17 19
[5,] 29 31
[6,] 41 43
[7,] 59 61
[8,] 71 73

Answer 2

Firstly, (23,29) is not twin prime.
Secondly, your answer may be found in here
Edit: I've tried your code, I found that length of Era is 23.
Maybe when running if (Era[i] + 2 == Era[i+1]), it reaches to 24 and causes the problem.

Answer 3

for (i in Era) will set i to 2, then 3, then 5 etc which is not what you intended. Use for (i in seq_len(length(Era) - 1)).
for (i in seq_len(length(Era) - 1)){
if (Era[i] + 2 == Era[i + 1]){
print(c(Era[i], Era[i + 1]))
}
}
#> [1] 3 5
#> [1] 5 7
#> [1] 11 13
#> [1] 17 19
#> [1] 29 31
#> [1] 41 43
#> [1] 59 61
#> [1] 71 73